Several DNA cloning systems employing the single-stranded filamentous phages as vectors have been developed. These have been used to clone fragments of DNA from the nematode Caenorhabditis elegans. These cloned fragments will be used as probes in an extensive, sensitive search for developmentally programmed rearrangments in the genomes of the nematode somatic cells. The method of detection involves forming duplexes between somatic DNA and individual cloned fragments and cleaving the hybrid molecules with S1 nuclease in order to detect points of mismatch resulting from DNA rearrangements. Using model rearrangement constructed by in vitro techniques, we have begun to validate this method of detecting rearrangements.